1. Field of the Invention
The present invention relates to a method for manufacturing an exhaust pipe assembly and an apparatus used for the method, and more particularly, to an apparatus for fixing a mounting ring to the exhaust pipe assembly by seam welding.
2. Description of the Related Art
An exhaust pipe assembly used for a vehicle such as a two-wheeled motor vehicle, comprises a plurality of (e.g., four) exhaust pipes, which are each formed by fixing a mounting ring to a first end of a pipe and inserting a mounting flange from a second end of the pipe. Each exhaust pipe is mounted with its first end communicated with an engine, and all the exhaust pipes converge on, for example, a single pipe at their second ends to be connected with a muffler or silencer.
For example, a MIG (Metal Inert Gas) welding or TIG (Tungsten Inert Gas) welding is employed for a method of fixing the mounting ring to the first end of the pipe. However, when the mounting ring is fixed to the first end of the pipe by means of MIG welding or TIG welding, a welding bead B protrudes from an inner circumferential surface of the exhaust pipe as shown in FIG. 7, so that the welding bead B behaves as resistance against an engine exhaust emitted through the exhaust pipe assembly. Such resistance against engine exhaust causes decrease in exhaust efficiency, eventually leading to the decrease in power of engine.
To address such problem, Japanese Unexamined Patent Application Publication No. 2000-97022, for example, discloses a method for fixing a mounting ring to a first end of a pipe by seam welding. As shown in FIG. 8, a seam welding apparatus 100 according to the JP2000-97022 comprises an outer circumferential electrode wheel 101 abutted on an outer circumferential surface of a mounting ring a, a pressing means 102 for pressing the outer circumferential electrode wheel 101 to the mounting ring a, an inner circumferential electrode wheel 103 abutted on an inner circumferential surface of a pipe b making up an exhaust tube, a transformer 104 serving as a voltage supply means for supplying a predetermined voltage to the outer circumferential electrode wheel 101 and the inner circumferential electrode wheel 103, and a controller 105 controlling the voltage of the transformer 104. The outer circumferential electrode wheel 101 and the inner circumferential electrode wheel 103 are rotatably provided with a predetermined axis-to axis distance D kept therebetween.
At first, an outer circumferential portion of the first end of the pipe b is inserted into the mounting ring a, so that the pipe b with the mounting ring a is arranged in between the outer circumferential electrode wheel 101 and the inner circumferential electrode wheel 103. Then, the pressing means 102 is allowed to press the outer circumferential electrode wheel 101 toward the mounting ring a, and under that condition, the voltage supply means supplies a voltage to the outer circumferential electrode wheel 101 and the inner circumferential electrode wheel 103. At the same time, both the inner and outer circumferential electrode wheels 101 and 103 are rotated in cooperation with each other to thereby perform the seam welding along entire circumferences. In this way, the mounting ring a is fixed to the first end of the pipe b.
According to the conventional seam welding apparatus 100 in FIG. 8, the diameter of the inner circumferential electrode wheel 103 is allowed to be smaller than that of the outer circumferential electrode wheel 101 so that the mounting ring a can be fixed to the pipe b having a smaller diameter with the axis-to-axis distance D between the outer and inner circumferential electrode wheels 101 and 103 being kept constant.
However, when the welding according to the JP 2000-97022 was performed and then a welding surface after the seam welding was observed, it was found that the outer surface (seam-welded outer surface) W1 of the exhaust pipe melted more than the inner surface (seam-welded inner surface) W2 of the exhaust pipe. Thus, it is recognized that there was a difference in welding temperature between the seam-welded outer surface W1 and the seam-welded inner surface W2, as illustrated in FIGS. 9A, 9B and 9C. This temperature difference causes locations of nuggets and coronas made by the seam welding to be displaced from the boundary between the pipe and the mounting ring toward the mounting ring. Furthermore, coronas are displaced even more toward the mounting ring because the thickness of the mounting ring is large, so that there is a problem that a sufficient welding strength is not able to be obtained.
The term “nugget” used herein means a welded region that has been subjected to weld solidification in a resistance welding process. Also, the term “corona” means a ring-like region formed around a nugget by a solid-phase welding in a resistance welding process.
The above welding temperature difference is considered to be attributable to an area of contact between the mounting ring a and the outer circumferential electrode wheel 101 being smaller than that of the pipe b and the inner circumferential electrode wheel 103, as shown in FIG. 10. More specifically, an amount of the current flowing per unit area is greater between the mounting ring a and the outer circumferential electrode wheel 101 than between the pipe b and the inner circumferential electrode wheel 103, so that an amount of heat generation by resistance between the mounting ring a and the outer circumferential electrode wheel 101 is also greater between the mounting ring a and the outer circumferential electrode wheel 101, and for this reason, the seam-welded outer surface W1 is considered to have melted more seriously than the seam-welded inner surface W2.
Therefore, in order to form a corona in the boundary between the mounting ring a and the pipe b. it is imperative to boost a voltage to be supplied to the outer circumferential electrode wheel 101 and the inner circumferential electrode wheel 103, i.e., to raise a welding temperature by increasing the current. To achieve this, however, it is not desirable to use a thick mounting ring from a viewpoint of efficiency. In addition, in the case of using materials whose surfaces are easy to melt, maintenance of electrodes becomes necessary, and thus it is not desirable.